High pressure access fittings are commonly employed in structures such as high pressure tanks, vessels, pipes or the like, for example, to insert probes, probe anodes, cupons, to take samples, and for many other purposes. High pressure access fittings are designed to permit safe, easy insertion and retrieval of corrosion and erosion monitoring systems as well as preventive maintenance devices for injecting inhibitors or for sampling, etc. even while under full operating pressure.
Such structures may be designed to contain fluids or materials at very high pressures such as 6000 psi (2,908,000 N/M.sup.2) for example, and at elevated temperatures. Many of the materials or fluids may be corrosive. The high pressure access fittings may be incorporated as part of the structure when built, or may be added to the structure later, and even while the structure or vessel is under pressure.
Special tools which include high pressure valves are employed to insert or remove the various tubes, probes or plugs. An example of one such tool is shown in applicants prior U.S. Pat. No. 4,537,071. Such tools can also be used to hot tap the vessel or structure after the fitting is installed and while the structure is under pressure.
The fitting usually includes a body welded or otherwise secured to the wall of the structure. The interior of the body of the fitting at the neck includes precision threads into which the plug or probe, for example, is secured. Just axially inside the threads is a seating surface for seals of the plug or probe. The exterior of the neck is provided with somewhat coarser threads to secure a cap, or quickly to land the tool to remove and insert the plugs or probes, or to drill the hole within the fitting after the body is secured to the wall.
The internal threads usually have a very shallow low pitch angle so that the tool is required to turn the plug or probe perhaps three or more turns to let the pressure bypass when the plug or probe is removed or backed out. Although such threads and seal seating surfaces may not normally be exposed to the corrosive contents of the structure, they are, however, exposed during installation or change out. Even after equipment is installed, the corrosives of the vessel or structure will remain in this area.
The body of the fitting must normally be the same or similar material as the structure wall to which it is attached to enable proper field attachment through welding. This usually requires that the body be made from a welding compatible grade of steel.
While the design of the body can increase the wall thickness to allow for certain corrosion over time, the internal threads and seal seats are critical and the thread configurations and diameters must be maintained for the access fitting to function as intended over the life of the structure. A failure in the area of the threads or seal seating surfaces brought on by corrosion may result in a violation of vessel or structure integrity which is a real safety and environmental concern. If the fitting threads corrode and become unusable, the fitting is no longer usable for access and should be removed or plugged, and replaced. Too many access fittings may compromise the structure. Accordingly, if a different metal can be employed for the internal thread section at the neck of the fitting which would be resistant to corrosion and help prevent galvanic corrosion, the service life of the fitting could be greatly extended. If the different metal is in the form of an insert, the insert must be secured in a proper and economical matter to ensure that the threads are axially aligned and correct, and that additional points of corrosion are not created.